Conventionally, a photovoltaic power generation cell (also called a ‘solar cell’ or a ‘cell’) is produced from a rectangular polycrystalline silicon wafer that has been sliced from a rectangular parallelepiped polycrystalline silicon ingot. Compared with solar cells employing polycrystalline silicon or amorphous silicon, solar cells employing a semiconductor single crystal silicon wafer have higher energy conversion efficiency, and since the cost of single crystal silicon wafers is becoming relatively low, they will be the mainstay of solar cells that will be widespread in the future. In this case, the solar cell is produced using a disk-shaped wafer that has been sliced from a cylindrical ingot.
For example, a single crystal wafer obtained by slicing a semiconductor single crystal ingot obtained by the Czochralski method (hereinafter, simply called the ‘CZ method’) or the floating zone melting method (hereinafter, simply called the ‘FZ method’) has a disk shape. A high module packing ratio, where the module packing ratio is defined as the proportion of the total area of photovoltaic power generation cells relative to the plane area of a photovoltaic power generation module (hereinafter, also called a ‘solar cell module’ or a ‘module’), cannot be achieved if solar cells are merely arranged on the plane without being changed in shape from their disk form.
In order to improve the actual energy conversion efficiency based on the plane area of the module, it is necessary to improve this module packing ratio. As a technique for increasing the module packing ratio, there is a generally well-known method in which cells are machined into a rectangular shape and made into an array. However, this method has the problem that, since the disk-shaped semiconductor single crystal wafer is cut into a rectangular shape, loss of arcuate-shaped crystal occurs.
As a technique that can solve the two problems of a low module packing ratio and crystal loss, a method has been disclosed in which a solar cell module is produced using arcuate cells formed when a square cell is cut out from a disk-shaped solar cell as disclosed in, for example, WO03/073516A1. Furthermore, a pattern that seems to be formed by arraying arcuate cells remaining after cutting out so-called pseudo-square cells has been disclosed on the internet, for example, in an article titled “Microsol Mono-crystalline Solar Cells” of unknown publication date and unknown author (Microsol Power (P) Ltd.; retrieved on Aug. 4, 2004) that can be located at <URL:http://www.microsolpower.com/home.asp>.
On the other hand, one crystalline silicon photovoltaic power generation cell generates a voltage of about 0.5 V in an operational state, regardless of its area. When currently popular 15 cm square cells are used, in order to generate a voltage of about 210 V, which is the input voltage of a widespread inverter for converting DC to AC, it is necessary to employ a photovoltaic power generation system in which a total of 420 cells are connected in series. When formation of an interconnectable module is attempted only from 15 cm square cells, the area of the module becomes about 12 m2, and it is not practical to produce such a huge module and install it on a roof, etc. A method is therefore employed in which, for example, twelve 1 m square modules are connected in series. Even if subdivided modules are installed in this way, when installation of a photovoltaic power generation system having a unit total area of 12 m2 is attempted on a roof, a dead space (non-occupied area) in which no module can be placed is often formed. Moreover, the weight of one module is as heavy as 15 kg, thus making installation on a roof difficult.
When cells were first produced, they were 10-cm square, then 12.5-cm square, and the 15-cm square cell is currently dominant. The productivity has increased by 2.25 times due to the increase in cell area, and 20-cm square cells are currently produced. However, in order to further popularize photovoltaic power generation systems and make use of single crystal silicon solar cells effectively, it is desirable to improve the proportion of the total module area relative to the effective area for installation on a south-facing roof (module coverage ratio, hereinafter also called simply ‘coverage ratio’) and simplify the installation procedure for a photovoltaic power generation system.